(1) Field of the Invention
The present invention relates to a novel thermoplastic resin composition having an excellent impact resistance.
(2) Description of the Related Art
Impact-resistance resins represented by ABS resins and high-impact polystyrene are ordinarily prepared by graft-polymerizing styrene, acrylonitrile, methyl methacrylate or other monomers to a rubber component. The physical properties of the final resin compositions are greatly influenced by the composition and structure of the graft copolymer, the content of the rubber component, and the polymerization method. Especially where a monomer is graft-polymerized on a rubber component according to the emulsion polymerization method, as is well-known, the particle size of the rubber component as the substrate has a great influence on the impact resistance and processability of the final resin composition. Namely, as the rubber particle size is increased, the impact resistance and processability of the obtained resin are improved.
Various methods for increasing the rubber particle size as much as possible have heretofore been proposed. For example, a resin composition having a good impact resistance has been proposed which is obtained by polymerizing a mixture comprising at least one monomer selected styrene, acrylonitrile, and methyl methacrylate, and monomers copolymerizable therewith, and having a group CH.sub.2 =C in the presence of a large-particle-size rubber latex obtained by enlarging a diene rubber with a copolymer latex formed from an unsaturated acid monomer and an alkyl acrylate (see Japanese Unexamined Patent Publication No. 58-63713). However, where a rubber component comprising butadiene units is used as the main structural units, the initial impact resistance of the obtained resin composition is excellent, but the resin composition has poor weatherability and low heat-resistant stability. If a styrene/butadiene copolymer is used as the rubber component, the styrene units reduce the rubbery characteristics of the copolymer, and therefore, the impact resistance of the obtained resin composition at low temperature is poor.
A resin composition comprising a graft copolymer obtained by enlarging a rubber only with an acid group-containing copolymer has excellent impact resistance, but the composition has a low enlarging efficiency and the molded article obtained as the final product has poor transparency and does not have an attractive appearance.